The invention relates generally to a method and apparatus for correcting color saturation in an electronic image processing system, in which a two-dimensional original image is electro-optically scanned in three primary colors by rows and columns, and, more particularly, to a method and apparatus in which the image signals are transformed to a luminance signal Y and two color difference signals or chrominance signals C1, C2 and the luminance signal Y is modified by electronic contrast processing of the image.
The aformentioned transformation is known in principle in the video art. There it is directed primarily to using electronic means to undertake color corrections if reception-induced color errors are present (e.g. color distortion) or if, during transmission, false color casts arise, which must be compensated. In electronic image processing, one often wishes to enhance the color saturation and color contrast of an image or to optimally match these to the characteristics of a photographic recording medium. The fundamentals of electronic color correction are described in, for example, the books Farbmetrik und Farbfernsehen (Color Measurement and Color Television), by H. Lang, pages 326 to 334 and 431ff, R. Oldenbourg-Verlag publishing house, Munich/Vienna 1978, and Digital Image Processing, by W. K. Pratt, particularly pages 50-90 and 155-161, John Wiley & Sons, New York/Chicester/Brisbane/Toronto, 1978.
Contrast valuation or weighting in the luminance channel is carried out in electronic image processing, in order to match the gradation of the overall transmission system to the photographic recording medium (global contrast valuation) and to enhance the contrast in specific image portion (local contrast valuation). A sharpening of the image can be achieved in this manner. The fundamentals of these methods are described in detail in the books Digital Bildverarbeitung, by F. M. Wahl, Springer-Verlag publishing house, Berlin/Heidelberg/New York/Tokyo, 1984, and Digital Image Processing, by W. K. Pratt, (loc. cit., see above).
In the reproduction of colored original images (positive-positive) or the manufacture of color photographic positive pictures from color negative originals, greater and greater reliance is being placed on electronic image processing and the use of color correction circuits. See, for example, European Patent Applications Nos. 70 680, 131 430, and 168 818. European Patent Application No. 168 818 corresponds to U.S. Pat. No. 4,661,843. Fundamentally, the original image is sampled (scanned) along lines and columns, and the resulting image signals are modified according to specific criteria. The image signals defining one image or frame are, as a rule, digitized and may be placed in a digital memory or in intermediate storage. The sampling of the original image is normally carried out serially for the three primary colors red, green blue (RGB).
Experience has shown that, for production of optimal positive images, the following image parameters must be adjusted or be capable of being adjusted:
(a) color balance, PA1 (b) color saturation, PA1 (c) contrast (gradation).
There is often the difficulty that these parameters cannot be adjusted independently of one another. When one adjusts the gradation, one finds that, for example, the color saturation has changed. For this reason, in video technology the RGB image signals are transformed into one brightness signal and two chrominance signals containing only color information. In electronic image processing, it has been found that contrast processing in the luminance channel can cause an influence on the color saturation in the chrominance channels. This color saturation shifting cannot be tolerated when image quality requirements are high. In particular, it has been observed that, upon enhancement of the brightness (high amplification of the luminance signal), a desaturation of the image occurs, while, upon reduction of the brightness, the image appears more strongly saturated.